Closures in Python

A question came up on Stack Overflow about nested functions and closures.

According to wikipedia, 'a closure is a first-class function with free variables that are bound in the lexical environment. Such a function is said to be "closed over" its free variables.' An example should make it clearer.

def adder(n): def func(x): return x + n return func

adder is a so-called factory function. We exercise it like so:

>>> i = 2 >>> f = adder(i) >>> f(10) 12 >>> j = 3 >>> g = adder(j) >>> g(10) 13

So the question on SO was, "why does the nested function" (func, assigned to f) "remember the first value" of n?

One answer is: because that's the rule!

>>> hex(id(i)) '0x100202810' >>> f <function func at 0x100479050> >>> f.func_closure (<cell at 0x10048fde0: int object at 0x100202810>,)

f.func_closure is a tuple. The first item is a "cell" that contains the same int object passed to adder in constructing f.

>>> hex(id(f.func_closure[0].cell_contents)) '0x100202810'

More about function objects here